The present invention relates to an air-fuel ratio control system for an internal combustion engine.
In general, a desired air-fuel mixture must be rich at the engine starting when it is cold, or during the warming up thereafter because the engine stability must be obtained. After the warming up, the air-fuel mixture can be maintained lean.
In a prior art electronic control type carburetor, a conventional choke valve mechanism is provided in the carburetor to enrich the air-fuel ratio when the engine is cold. Otherwise, the feedback control for the electronic control type carburetor could not be carried out to stabilize the engine operation. For example, the output of an O.sub.2 sensor is not enough because the exhaust temperature rises insufficiently.
According to such an choke valve mechanism, the actual air-fuel ratio is determined by the combination of the vacuum produced by the air flow through the venturi and the vacuum produced by the throttle resistance of the choke valve. In addition, the turbulent flow of the air caused by the choke valve passes through a nozzle portion. Thus, the vacuum near the nozzle portion is not necessarily in proportion to the actual air flow. For such a reason, the actual air-fuel ratio cannot become the desired air-fuel ratio so that a lot of CO, HC and the like in the exhaust gases are exhausted into the atmosphere. This results in the fuel cost increase.